Prevent prerender from intercepting error code responses

We need to stop the prerender.io middleware package from intercepting errors (3xx, 4xx, 5xx) and forwarding them through prerender.io.

The way how it's currently implemented it intercepts every response and forwards it through prerender.io. This caused us to rack up additional fees this last month. So it would be great to make that configurable.

Also the list of bot user-agents is hard-coded. So, we are not able to configure this, which is what we would "like" to be able to do.